This invention relates generally to cardiac stimulators and more particularly to cardiac pacemakers.
Over the years, a variety of pacing modes have been developed to respond to changes in spontaneous heart activity. One of the earliest of these is disclosed in U.S. Pat. No. 3,857,399, issued to Zacouto. Zacouto provides a system in which the underlying heart rhythm is used to control the onset of pacing, but not the pacing rate. The onset of pacing in Zacouto can occur after an interval either shorter or longer than the underlying physiologic interval.
U.S. Pat. No. 4,163,451 issued to Lesnick et al discloses a pacemaker having an overdrive pacing mode. It provides for initiation of cardiac pacing at an interval shorter than the detected cardiac interval. The stated purpose of this pacing modality is to provide a method of tachycardia treatment. Following onset of pacing, the pacing rate in Lesnick is sequentially decremented. Entry into this pacing mode at exit from its pacing mode are determined by tachycardia detection and termination criteria, so it should be expected that the overdrive pacing mode in Lesnick would be employed only occasionally.
U.S. Pat. No. 4,562,841 issued to Brockway et al discloses a dual chamber pacemaker, in which gradual increase and decrease of pacing intervals occurs in response to changes in underlying heart rate. However, adjustment of the pacing interval is primarily based on the atrial rate, rather than the ventricular. There appears to be no provision for adjusting the escape interval as a function of the measured escape interval preceding a PVC.
U.S. Pat. No. 4,503,857 issued to Boute et al discloses another cardiac pacemaker which varies its escape interval in response to the rate of underlying heart activity. This modality is referred to as "flywheel" pacing, and is intended to prevent abrupt changes in pacing rate. The system allows the pacing rate to vary with the underlying rate, but only within narrow limits. This system is designed specifically to prevent a rapid increase in pacing rate in response to a PVC or other abrupt change in heart rate.
U.S. Pat. No. 4,467,810 issued to Vollmann discloses a dual chamber pacemaker which employs a fall-back pacing mode in which ventricular pacing intervals are gradually incremented in response to a high atrial rate. The alteration of escape intervals is intended to terminate atrial tachycardias.